Recently, chromatographic separation of enantiomers from their racemic mixture using a chiral stationary phase has received considerable attention, arising in large part from the availability of relatively inexpensive chiral stationary phases. The stationary phase generally consists of a chiral organic moiety, as for example cellulosic materials, attached to an underlying inert core support such as silica. It was believed that large pore silicas (e.g., pore sizes larger than about 500 angstroms) were necessary for an efficient and effective chiral stationary phase, and in fact, silicas with a pore size greater than about 1000 angstroms have been favored as the inert core support. To our surprise, we have found small pore silicas, i.e., silicas with a mean pore size less than about 100 angstroms, to be quite effective in separation of enantiomers using a chiral stationary phase using simulated moving bed chromatography. Although Grieb et al., Chirality, 6, 129 (1994) recently have shown that silicas of 120 angstrom pore size can function well in analytical columns we have observed some quite unanticipated properties which make even smaller-pore size silicas (50-100 angstrom pore size) still more desirable for use in simulated moving bed chromatography.
To better understand our invention in the context of theory and conventional practice it will be helpful to briefly review some of the principles of liquid chromatography most relevant to our invention. One fundamental property in liquid chromatography is k', the capacity factor, which is defined as EQU k'=n.sub.s /n.sub.m ( 1)
where n.sub.s is the total moles of material being separated in the stationary phase and n.sub.m is the number of moles of this material in the mobile phase. Where there are several components present, the capacity factor for the ith component is EQU k'(i)=n.sub.s (i)/n.sub.m (i)
It is clear from this expression that the larger is k' the greater will be the capability of the stationary phase to absorb the component, but also the larger will be the amount of solvent to totally desorb it.
The retention time, t.sub.r, for component i, t.sub.r (i), is related to the time it takes for the mobile phase to travel the length of the column, t.sub.0, by the distribution of component between the stationary and mobile phases according to the equation, ##EQU1## Thus, the capacity factor k' also is related to the relative retention time of the components in question.
For two components, the ratio of their relative retention times, .alpha., is ##EQU2## where .alpha..sub.ij is the selectivity factor between components i and j. In traditional analytical chromatography one desires to maximize the selectivity so as to effect complete separation of the components. Finally, the volume, V.sub.r, of the mobile phase required to elute a component as measured to the apex of the peak is related to the flow rate, F, of the mobile phase and retention time of the component by, EQU V.sub.r (i)=t.sub.r (i)F
from which it follows that ##EQU3## Thus, classical liquid chromatography theory as supported by much experimental evidence leads to the conclusions that the retention volume of a particular component, relative to the retention volume of the pure mobile phase, depends only on the capacity factor for the component, although relative retention volumes and relative retention times for two components depend only on the ratio of the two capacity factors, and it is the ratio of the capacity factors which define selectivity.
One form of chromatography well adapted to continuous, commercial-size separation is simulated moving bed chromatography. In continuous moving bed chromatography the stationary phase moves relative to the feed and mobile phase inputs, and the extract and raffinate outputs. Because of the difficulty of implementing a moving stationary phase in chromatographic separations its simulation is favored in practice (hence the name simulated moving bed chromatography) where incremental positional changes of the input and output streams, relative to a static stationary phase, is effected at regular intervals. Although many of the foregoing relations apply to simulated moving bed chromatography some additional nuances are applicable when the separations are of chiral substances using conventional chiral stationary phases.
The conditions in simulated moving bed chromatography can be significantly modified from those required for analytical and batch mode preparative chromatography. In particular, the separation of enantiomers from their racemic mixture using a chiral stationary phase in simulated moving bed chromatography can be performed effectively at low values of k' (low capacity factor), thereby minimizing the amount of mobile phase which is needed. Since an appreciable cost of the separation process is associated with the mobile phase and its recovery from the raffinate and extract streams, operation at a low capacity factor affords substantial cost savings accruing from a lower mobile phase inventory, lower utility costs in recovering the mobile phase, and other ancillary costs. But inquiry does not end here|
Recently we have evaluated the effects of several chromatographic variables on the cost of effecting chiral separations by simulated moving bed chromatography. One result is that separation costs decrease with an increase in the capacity factor, i.e., k'. For a series of materials tested under identical conditions the magnitude of the capacity factor, k', is an indication of the capacity of the stationary phase. At first this may appear counterintuitive, since increasing k' translates to increased amounts of mobile phase needed to desorb the component, and attending increases in solvent recovery costs. However, larger values of k' permit adjustments of other variables which ultimately lead to lower costs. The essential feature to focus on is that operational costs of chiral separations via SMB decrease with increasing k', a conclusion which is not apparent and may well be unexpected.
The so-called "breakthrough test" is a measure of the column capacity of a stationary phase. In a breakthrough test a feed of a specified concentration is passed over the stationary phase and the effluent is collected and analyzed. The relative concentration of each component in the effluent is plotted relative to the effluent volume and the appearance of each component is a measure of the column capacity of the stationary phase.
Our invention arises from the totally unexpected observation that as the pore size of silica is varied the capacity factor varies inversely with pore size. That is, as the silica pore size decreases, the capacity factor, k', of the chiral stationary phase for which silica is used as an inert core support increases. This, too, is counterintuitive, for the chiral organic moiety generally has a high molecular weight and would not be expected to utilize the small pores of the silicas of our invention. Nonetheless, the experimental observations are unequivocal and unassailable; chiral stationary phases having small pore silicas as an inert core support demonstrate higher capacities than analogous chiral stationary phases with large-pore silicas as the inert core support.
According to its definition, the capacity factor also is a measure of the quantity of material which the adsorbent can hold, and in SMB this is related to the capacity of the column. Column capacity is related to both the productivity of the SMB process as well as the number of active sites of any adsorbent. In general, the higher the productivity of an adsorbent, the less solvent is used, and the less solvent recovery is needed. To our surprise, we found an increase in column capacity in going from the conventional large pore silicas previously used to the 50-100 angstrom silicas of this invention. Consequently, use of such small-pore silicas as the support in CSPs for SMB chiral separations provides unexpected benefits over the use of traditional large-pore silicas, and therein lies our invention.